Hydrogen Ions & pH (Edexcel GCSE Chemistry: Combined Science)

• We have already seen that acids are substances that contain hydrogen ions in solution
• The more hydrogen ions the stronger the acid, but the lower the pH
• The higher the concentration of hydroxide ions in a solution the higher the pH
• So pH is a measure of the concentration of H⁺ ions in solution, but they have an inverse relationship

The pH scale is logarithmic so that each change in pH is a tenfold change in hydrogen ion concentration

• The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10
• Therefore an acid with a pH of 3 has ten times the concentration of H⁺ ions than an acid of pH 4
• An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H⁺ ions than an acid with a pH of 4
• From this we can summarize that for two acids of equal concentration, where one is strong and the other is weak, then the strong acid will have a lower pH due to its capacity to dissociate more and hence put more H⁺ ions into solution than the weak acid